Environmental Engineering Reference
In-Depth Information
2.3.4.3 Specific efficiencies
Generally, the efficiency of a process can be arbitrarily defined to expose the most
important aspect. For example, the exergy effect of the hot water heated in pipe by
solar radiation can be related either to the exergy of heat
q
at the temperature of the
Sun's surface
T
Sun
;
q
T
0
/T
Sun
), or to the exergy
b
Sun
of the Sun's radiation, or to
the exergy of heat
q
absorbed at the water pipe temperature
T
W
;
q
·
(1
−
T
0
/T
W
). The
exergy efficiency increases successively through the above three possibilities due to the
decreasing values of the denominators in the efficiency formulas:
q
·
(1
−
·
(1
−
T
0
/T
Sun
)
>
b
Sun
> q
T
0
/T
W
). An exergy efficiency which relates the process effect to the
decrease of the Sun's exergy,
q
·
(1
−
T
0
/T
Sun
), is unfair because the exposed surface of
pipe obtains only the solar radiation exergy and the pipe is independent of irreversible
emissions at the Sun's surface. Relating the process effect to the exergy of heat absorbed,
q
·
(1
−
T
0
/T
W
), favors the exposed surface by neglecting its imperfection during the
absorption of heat
q
. Thus, from these three possibilities, the relating the heating water
effect to the exergy
b
Sun
of the Sun's radiation is the only best estimation in this analysis.
·
(1
−
2.3.4.4 Consumption indices
Sometimes instead of efficiency the specially defined indices are used for estimation
of processes. For example there are some processes which occur spontaneously due
to interaction with the environment. Drying, cooling, vaporizing or sublimation, are
the examples of such processes in which the self-annihilation of exergy takes place.
Often these processes, especially in industrial practice, are accelerated with use of the
appropriate driving input. Exergy application for estimation of perfections of these
processes reveals some problems.
For example applying the common exergy efficiency definition, effect and input
ratio, leads to a negative or infinite value of the efficiency. Therefore instead of the effi-
ciency some specially defined criteria have to be used for the evaluation and comparison
of processes perfection. For example for drying processes the unit exergy consumption
index is defined as the ratio of the exergy of the used in the drying medium to the
mass of the liquid extracted in form of the vapor. In the case of the application of solar
energy for drying, the index would express the exergy of absorbed radiation per mass
of the vaporized moisture.
Another index can be used for process occurring in a water cooling tower. Szargut
and Petela (1968) propose the evaluation of the process with the index defined as ratio
of the sum of exergy lost in the tower and the heat extracted from water. The typical
value of the index for cooling tower of steam power station is about 0.088 kJ of exergy
per kJ of heat.
Petela (1990) proposes a specific approach to the exergy annihilation due to spon-
taneous processes. He considers the natural exergy annihilation rate which expresses
the ability of the environment to spontaneously reduce the exergy of a substance or
radiation. The natural wind velocity, the temperature and composition of environment
air, particularly humidity, as well as the solar radiation, the local surrounding surfaces
configuration and surfaces' emissivities taken into account together can determine the
available exergy effect for annihilation of exergy in the spontaneous processes of dry-
ing, cooling, etc. A so called “windchill'' factor is the example of the concept expressing
certain ability of environment air.
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